1. Technical Field
An inductively coupled plasma source of a plasma reactor generally includes an antenna having plural conductive coils and an RF power generator coupled to the antenna.
2. Background Discussion
An inductively coupled plasma source is employed in many plasma reactors for performing plasma-enhanced processes, such as etching, on a workpiece such as a semiconductor wafer. An inductively coupled plasma source includes one or more RF-driven inductive coils to deliver power to a plasma. The coil antenna may include two or more coils in order to control radial distribution of plasma ion density. A typical coil antenna may be a flat spiral conductor or a cylindrically shaped spiral winding. The inductance of such a coil antenna is typically very high, causing the voltage on the RF power terminal of the coil antenna to be very high. As a result, it is often necessary to connect a capacitor in series with the coil antenna in order to reduce the voltage on the RF power terminal. One problem is that the introduction of such a series capacitor reduces the efficiency of the coil antenna.
Another problem with a typical inductively coupled plasma source is that the use of two or more coils in the antenna creates an M-shaped radial distribution of plasma ion density, in which there are pronounced minima at the wafer edge and at the wafer center. This problem arises from interaction or mutual coupling between the different coils of the antenna.